I think that the bug-list is not a very good indication at how close to complete Leopard is. The majority of the bugs in Leopard may be on that list, and Apple may already know how to fix them. Or, Apple may have already fixed them:
Apple may have several different branches for Leopard, perhaps even separate ones for bug-fixing, features, and the developer preview. The builds that we are seeing are meant to give developers access to APIs so that they can try out new features. These builds do not necessarily have all that much in common with the actual new features which may be being added in Leopard.
For instance, a new Finder does not require developers to see it; what features in it will they need to see, unless they need to design plug-ins for it? In fact, the only plug-ins which they may need to make for a new Finder are plug-ins for QuickView, which of course they can already make for Spotlight and the current finder!
A new UI is basically just a theme change, and thus would require very little if any advanced developer notice. The programs should theoretically "just work" with the new theme.
Any other "secret features" may be along the same lines - features that don't do anything to help developers, but which really make users happy.
In fact, the features that Apple has demonstrated so far are probably not supposed to be Leopard's killer-features; it is not a good idea to announce the best features nine-to-twelve months ahead of when the user will see them. To give away all the features before then would be idiotic. (I hope Apple is not idiotic.) The real question is, can we think of any killer features we would like? What might Apple be implementing?
What we saw in WWDC06 was probably to hold us over until details on Leopard are actually released (unleashed?). I think Apple is probably doing the same thing with the iPhone, at least in software, and perhaps in hardware (they may implement 3G and/or allow for widget development for the iPhone through Dashcode, etc, which would by leaps and bounds extend the iPhone's already very useful usefulness).
I do not think Apple is holding back features because of Microsoft - more likely, they are holding them back for the marketing sense of doing so. Naming the features closer to the product's release will generate more hype.
Hopefully, I'm right!